1. Field of the Invention
This invention relates generally to a system and method for determining if a relative humidity (RH) sensor that measures the relative humidity of cathode inlet air provided to a fuel cell stack or a high frequency resistance (HFR) measuring circuit that measures stack water content is operating properly and, more particularly, to a system and method for determining whether an RH sensor or an HFR circuit is operating properly by determining whether the output signal from the RH sensor or the HFR circuit is valid.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated at the anode catalyst to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons at the cathode catalyst to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically, but not always, include finely divided catalytic particles, usually a highly active catalyst such as platinum (Pt) that is typically supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
A fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow fields are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow fields are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
The membrane within a fuel cell needs to have sufficient water content so that the ionic resistance across the membrane is low enough to effectively conduct protons. Membrane humidification may come from the stack water by-product or external humidification. The flow of reactants through the flow channels of the stack has a drying effect on the cell membranes, most noticeably at an inlet of the reactant flow. However, the accumulation of water droplets within the flow channels could prevent reactants from flowing therethrough, and may cause the cell to fail because of low reactant gas flow, thus affecting stack stability. The accumulation of water in the reactant gas flow channels, as well as within the gas diffusion layer (GDL), is particularly troublesome at low stack output loads.
As mentioned above, water is generated as a by-product of the stack operation. Therefore, the cathode exhaust gas from the stack will typically include significant water vapor and liquid water. It is known in the art to employ a water vapor transfer (WVT) unit to capture some of the water content in the cathode exhaust gas, and use that water content to humidify the cathode input airflow. Water in the cathode exhaust gas at one side of the water transfer elements within the WVT unit is absorbed by the water transfer elements and transferred to the cathode air stream at the other side of the water transfer elements. A typical WVT unit includes membranes made of a special material where the wet flow on one side of the membrane is transferred through the membrane to humidify the dry flow on the other side of the membrane.
As discussed above, it is generally necessary to control stack water content so that the membranes in the stack have the proper proton conductivity, but where the flow channels do not become blocked by ice if the water freezes during system shut-down. It is known in the art to provide an RH sensor in the cathode air inlet of a fuel cell system to measure the humidification of the cathode inlet gas stream as it enters the stack. Using the measured inlet relative humidity and the water species balance, or mass balance of water, the RH profile of the fuel cell system, including cathode air outlet flow, can be estimated. The ability of the RH sensor to provide an accurate reading of the RH is determined by the cost and complexity of the sensor. It is typical desirable to limit the cost of the sensor, which reduces its accuracy.
Another technique for determining stack water content is known in the art as high frequency resistance (HFR) humidification measuring, where high frequency in this context is typically 300 Hz-10 kHz. HFR humidification measurements are generated by providing a high frequency component or signal on the electrical load of the stack so that a high frequency ripple is produced on the current output of the stack. The resistance of the high frequency component is then measured by a detector, which is a function of the level of humidification of the membranes in the stack. High frequency resistance is a well-known property of fuel cells, and is closely related to the ohmic resistance, or membrane protonic resistance, of the fuel cell membrane. Ohmic resistance is itself a function of the degree of fuel cell membrane humidification. Therefore, by measuring the HFR of the fuel cell membranes of a fuel cell stack within a specific band of excitation current frequencies, the degree of humidification of the fuel cell membrane may be determined. This HFR measurement allows for an independent measurement of the fuel cell membrane humidification, which may eliminate the need for RH sensors.
Models are sometimes employed in fuel cell systems for determining the water content in the fuel cell stack. For example, it is known to employ a water buffer model that estimates the amount of water that is in the stack at any given time. Also, a water transfer model is known that estimates the water transfer in the WVT unit using the water buffer model. The water transfer model can estimate the cathode air inlet relative humidity and using that value and various operating parameters of the fuel cell stack, such as temperature, cathode stoichiometry, pressure, stack current density, etc., estimate the relative humidity of the cathode outlet gas by accounting for the water buffers within the fuel cell stack. The MEA and diffusion media within the stack have some water carrying capacity so changes in the above conditions do not immediately translate into the steady state value of outlet humidity. Using the estimation of the relative humidity of the cathode outlet gas and the water transfer capability of the WVT unit, the model then revises the estimation of the relative humidity of the cathode inlet air.
If the stack operating conditions require a different relative humidity for the cathode outlet gas, then the system control can change the temperature of the cooling fluid flowing through the stack to change the temperature of the stack, which changes how much water the cathode air can absorb. Particularly, if the stack temperature increases, the ability of the cathode air flowing through the stack to saturate with water increases where the absolute humidity of the cathode air may stay the same, but the relative humidity of the cathode air decreases.
The WVT unit degrades over time where its effectiveness to transfer water from the cathode outlet gas to the cathode inlet air decreases. This phenomenon may be the result of various things, such as membrane contamination, membrane degradation, etc. For those systems that employ an RH sensor between the WVT unit and the fuel cell stack and/or an HFR measuring circuit, the output of the sensor or circuit can be used to correct the water transfer model so that the estimate of the relative humidity of the cathode inlet air is adjusted as the WVT unit degrades. However, the RH sensor or HFR circuit themselves sometimes fail and/or drift where the sensor or circuit may not be giving an accurate reading of the water content of the cathode inlet air. In this circumstance, the model may be adjusted based on the inaccurate relative humidity measurement, causing stack operation performance problems. For example, if the RH sensor or HFR circuit is giving a measurement of the relative humidity of the cathode inlet air that is lower than the actual value, the water transfer model may adjust the temperature of the fuel cell stack lower, compensating for a perceived stack dry-out. This would cause the actual cathode inlet and outlet relative humidities to go up to a higher level than desired, which could cause stack instability as a result of flow channel flooding.
Additionally, other sensors or devices in the system may malfunction, such as coolant temperature sensors, cathode air flow meter, pressure sensors, etc. Therefore, the RH sensor or HFR circuit output may indicate WVT unit degradation, where the sensor measurement is accurate, but the system control may not interpret the change properly.
U.S. patent application Ser. No. 13/197,535, titled, Utilization of HFR-Based Cathode Inlet RH Model in Comparison to Sensor Feedback to Determine Failed Water Vapor Transfer Unit and Utilize for a Diagnostic Code and Message, filed Aug. 3, 2011, assigned to the assignee of this application and herein incorporated by reference, discloses a system and method to detect a crossover leak in a WVT unit using HFR and RH sensor measurements.